The present invention relates to a heat exchanger that utilizes vapor cooling to transmit thermal energy between two fluids.
In gas turbine engines, there is a need to reject heat from the engine during operation. Typically, that need has been addressed by transferring heat energy from a gas (e.g., hot air) to a liquid (e.g., engine fuel) through heat exchangers. Fuel acts as the principal heat sink on gas turbine engines for aircraft. However, there are limits on how much heat can be transferred to fuel before that fuel degenerates, and at about 218-232° C. (425-450° F.) at typical pressure conditions the fuel can instantaneously ignite (or auto-ignite). In order to prevent dangerous conditions associated with undesired auto-ignition of the fuel, known heat exchangers have been configured to provide buffer cavities.
FIG. 1 is a schematic view of a prior art heat exchanger 10 having a number of parallel buffer layers 12, hot air layers 14, and fuel layers 16. The layers are not shown to scale in FIG. 1. The heat exchanger 10 is configured such that the fuel layers 16 and the hot air layers 14 alternate, and a buffer layer 12 is located between adjacent fuel layers 16 and hot air layers 14. The hot air layers 14 and the fuel layers 16 are each passageways that allow hot air and fuel to flow through them, respectively. The buffer layers 12 are evacuated or partially evacuated voids that separate the fuel layers 16 and the hot air layers 14 to contain any leaks in the skins of those layers, which helps minimize the risk of fuel ignition due to interaction with the hot air. A series of pins (or fins) 18 extend across the buffer layers 12 between adjacent hot air layers 14 and fuel layers 16. The pins 18 provide structural support between layers of the heat exchanger 10, and also define paths for conductive heat transfer between adjacent hot air layers 14 and fuel layers 16 across the buffer layers 12. Substantially all of the heat transfer between different layers of the heat exchanger 10 occurs via conduction through the pins 18.
Known heat exchangers with buffer layers (see, e.g., FIG. 1) are relatively large and heavy, and heat transfer across the buffer layers is inefficient. In addition, because temperatures are not quickly and efficiently equalized across buffer layers, thermally-induced stresses can lead to undesirable heat exchanger lifespan reductions of known heat exchangers. Thus, it is desired to provide a relatively small and lightweight heat exchanger that can efficiently and reliably transfer heat between two fluids.